This invention relates to a vibration damping apparatus for damping vibrations in a vibrating system.
Hydraulic dampers are known for use in helicopter rotor systems for example, for damping helicopter blade movements in a plane of rotation as the blades rotate (so called xe2x80x9clead lag dampersxe2x80x9d), which vibrations can give rise to a phenomenon known as ground resonance, although similar hydraulic dampers are provided in other vibrating systems to damp vibrations.
In one form of damping apparatus, the apparatus includes a piston movable in a chamber in response to vibrations, piston movement being resisted by fluid pressure in the chamber at either side of the piston. Restricted fluid flow from one side of the piston to the other is permitted so that the piston may move in the chamber whilst providing damping forces to counteract such piston movements and provide damping.
Mechanical vibrating systems employing such hydraulic damping apparatus may experience modes of vibration other than at a fundamental frequency which primarily it is desired to damp. Such additional vibrations need not necessarily require damping, but the operation of a damping apparatus which responds to such additional vibrations may result in unnecessarily high damping forces which the surrounding structure will have to be strengthened to support.
In an effort to alleviate this problem it is known to incorporate into the damping apparatus a load limiting device such as a pressure relief valve, which limits the maximum damping force which can be provided by relieving fluid pressure at either side of the piston when high fluid pressures are produced. However such devices tend to degrade the ability of the damping apparatus to produce useful damping forces to damp the fundamental frequency, particularly in the presence of higher additional frequency vibrations since during part of each oscillation forces will be produced which assist the motion which it is intended to damp.
According to one aspect of the invention we provide a damping apparatus for damping vibrations in a vibrating system including a piston which is reciprocally movable in a fluid filled chamber, piston movement in either direction of movement being resisted by fluid pressure in the chamber behind the piston, which resistance provides damping forces which act to oppose piston movement, the piston being connected to one component of the vibrating system and the chamber being connected to a second component of the vibrating system, characterised in that means are provided to relieve the fluid pressure behind the piston to allow substantially unopposed piston movement in the chamber without damping, forces being provided during, piston movements which occur in response to vibrations in the vibrating system at frequencies other than a fundamental frequency which the damping apparatus primarily is intended to damp.
Thus in circumstances where the generation of unwanted damping forces in response to vibrations at frequencies other than a fundamental frequency it is desired to damp, can act in the same direction as the disturbing force giving rise to the fundamental vibrating frequency, damping forces are relieved. Thus the damping apparatus can be tuned to provide damping forces primarily in response to vibrations at the fundamental frequency.
In one embodiment the fluid pressure relief means includes a fluid by-pass means which has a first passage for fluid including a first one way valve means which permits substantially unopposed flow of fluid through the passage from a first side of the piston to an opposite second side of the piston, and a second passage for fluid including a second one way valve means which permits substantially unopposed flow of fluid through the passage from the second side of the piston to the first side, and a fluid control means which controls the flow of fluid through one or other of the first and second passages of the by-pass means depending upon the direction of the velocity of the fundamental vibrations frequency to be damped.
An actuating means may be provided to operate the control means in response to the direction of the velocity of the fundamental frequency vibrations to be damped and to changes in the direction.
Thus irrespective of the direction of the disturbing force giving rise to vibrations other than at the fundamental frequency, the fundamental vibrating frequency is damped at least when fluid flow through the by-pass means, is prevented.
The actuating means may include velocity direction responsive means, such as an accelerometer, for sensing the direction of the velocity of the fundamental frequency it is desired to damp.
Typically the damping apparatus includes a first restricted fluid flow path for fluid from a first side of the piston to a second side of the piston so that when fluid flow through the by-pass means is prevented, piston movement in the chamber in a first direction is permitted controlled by the rate of fluid flow through the first fluid flow path, and a second restricted fluid flow path is provided for fluid from the second side of the piston to the first side so that when fluid flow through the by-pass means is prevented, piston movement in the chamber in a second direction is permitted controlled by the rate of fluid flow through the second fluid flow path.
According to a second aspect of the invention we provide a vibrating system including a damping apparatus of the first aspect of the invention.
According to a third aspect of the invention we provide a helicopter including a rotor system having at least one damping apparatus of the first aspect of the invention.